With a high transmission speed and a large capacity in a network, a method of transmitting and receiving a signal using an optical transmission path has been spread in place of existing electrical wiring.
In order to perform optical transmission at a high speed, a light-emitting element such as a vertical cavity surface emitting laser (VCSEL) is driven at a high speed. A high-frequency signal is attenuated at a transmission path and the waveform is deteriorated, so that a pre-emphasis method of compensating for the signal on the transmission side beforehand has been used.
As a pre-emphasis type drive circuit, there has been proposed a circuit that combines a drive signal and the delayed signal and generates a drive waveform of the VCSEL (For example, see FIG. 11 of Japanese Laid-open Patent Publication No. 2012-080061). A pre-emphasis generation circuit that is used in such a drive circuit generates a pre-emphasis waveform that directly modulates the VCSEL at a high speed by using analog delay (variable delay). The generated pre-emphasis waveforms (a waveform that corresponds to an input drive signal and a waveform that corresponds to the delayed signal) are combined and a VCSEL drive signal is output. In a multiplex circuit, the collector of a transistor to which a drive signal is input and the collector of a transistor to which the delayed signal is input are connected to the common current source, and a synthetic drive signal is obtained from a common output end.
In the above-described multiplex circuit, there is a problem of variation in jitter of input waveforms. The jitter variation is attributed to the occurrence of a crosstalk between inputs of the multiplex circuit and variation in collector potentials because the collector ends of the transistors are connected to the common current source. This is a problem specific to a drive circuit that processes a high-speed signal.